Attempts have been made to provide flame spray apparatus which include an internal burner operating to produce an ultra-high velocity flame jet. One such ultra-high velocity flame jet apparatus is set forth in my earlier U.S. Pat. No. 2,990,653 entitled "METHOD AND APPARATUS FOR IMPACTING A STREAM OF HIGH VELOCITY AGAINST THE SURFACE TO BE TREATED" issued July 4, 1961. Such apparatus comprises an air cooled double or triple wall cylindrical internal burner whose interior cavity forms a cylindrical combustion chamber. Downstream of the point of initial combustion, the chamber is closed off by a reduced diameter flame jet nozzle.
In a further attempt to provide such ultra-high velocity flame spraying apparatus for metal, refractory material or the like, introduced to the high velocity flame spray stream in powder form or in solid small diameter rod form, an arrangement was devised utilizing a hot gaseous primary jet stream of relatively low momentum which fuses and projects a stream of molten particles into a second gaseous jet stream of lower temperature, but possessing a very high momentum. Such type of apparatus and method is set forth in U.S. Pat. No. 4,370,538 filed May 23, 1980, entitled "METHOD AND APPARATUS FOR ULTRA-HIGH VELOCITY DUAL STREAM METAL FLAME SPRAYING". The method and apparatus of that patent employs the first stream in the form of an oxy-fuel flame or an electric arc-producing plasma, while the second stream comprises a flame-jet produced by an air/fuel flame reacting at high pressure in an internal burner device. In combining the two streams, preferably the molten particles are carried by the first stream at relatively low velocity but relatively high temperature, while the supersonic jet stream which impinges the entrained molten particles against the surface to be coated at ultra-high velocity is discharged from an internal burner combustion chamber wherein combustion is effected at relatively high pressure. The second stream is directed through an annular nozzle surrounding the primary stream. Further, the primary and secondary streams are projected through a nozzle structure to the point of impact against the substrate to be coated as liquid particles travelling at supersonic speed, under the acceleration provided by the secondary jet of heated gas. In some cases, as in spraying of high temperature ceramics, the oxy-fuel flame may not be hot enough to provide adequate melting of the particles.
In conventional cold air powered abrasive blast (sand blast) equipment, it is usual to use an elongated nozzle made of extremely hard material such as tungsten carbide through which the abrasive particles are directed at supersonic velocity. The compressed air stream with entrained abrasive particles passes through such nozzle and is accelerated to peak velocities of about 100 meters per second. There is no need in such conventional cold air powered technology to confine the particle stream flowing through the nozzle bore. For such conventional apparatus, the particles strike the walls of the nozzle with little abrasion effect due to the choice of nozzle material.
When the accelerating compressed air stream is replaced by the hot products of combustion, of a like flow of compressed air, the available energy to accelerate the abrasive particles is increased about eightfold. Peak particle velocities over 300 meters per second are obtained. Such an impacting stream against the surface to be cleaned is several-fold more effective than that for its cold air flow counterpart, and additionally great economies of operation result.
In an effort to design reliable hot gas abrasive blast systems, many attempts have been made to use materials such as water-cooled tungsten carbide for the inner nozzle surface. However, it has been found impractical to prevent nozzle wear by such excessively hard metal. The carbide is heated to the point where it is eroded away by oxidation and additionally the material may crack badly.